Automatic phase control loop without false locks due to harmonics



Nov. 1, 1966 G. R. VAUGHAN 3,283,260

AUTOMATIC PHASE CONTROL LOOP WITHOUT FALSE LOCKS DUE TO HARMONICS FiledJune 30, 1965 2 Sheets-Sheet 1 Low FREQUENCY fl REFERENCE &

BUFFER f2 AMPLIFIER 22 IO [20 I8 l6 OUTPUT NO.| OSCILLATOR [F PHASEMIXER (f 4,) NO. I AMPLIFIER DETECTOR AMPLIFIER BUFFER AMPLIFIER ll 2lOUTPUT N02 oscILLAToR IF PRAsE MIXER P (f -2f N02 AMPLIFIER DETECTORAMPLIFIER BUFFER AMPLIFIER OUTPUT No.3 OSCILLATOR IF PHASE MIXER (f -3mNO. 3 AMPLIFIER DETECTOR AMPLIFIER FIG. George R. Vaughan,

INVENTOR. W WI BY W J. I

AT TORNEYS Nov. 1, 1966 G. R. VAUGHAN 3,233,260

AUTOMATIC PHASE CONTROL LOOP WITHOUT FALSE LOCKS DUE TO HARMONICS LOW 7f FREQUENCY REFERENCE I9 f FREEHLSSIEINCE BUFFER 2 REFERENCE AMPLIFIERF22 IO 20 Is IF OUTPUT OSCILLATOR MIXER AMPLIFIER PHASE '6 (f f I NO. II0 mc DETECTOR AMPLIFIER FIG. 2

SIGNAL INPUT f =50O /IO LOCAL I 20 OSCILLATOR 335 5 IO mc NOII MIXER 490me FIG. 4

--* MIXER FIG. 5

w MIXE :1 L George R. Vaughan,

1NVENTOR.

FIG. 6 M J lw,

ATTORNEYS 3,283,260 AUTOMATTC PHASE CONTROL LOOP WITHOUT FALSE LQCKS DUETO HARMONICS George R. Vaughan, Linthicum, Md., assignor, by mesneassignments, to the United States of America as represented by theSecretary of the Army Filed June 30, 1%5, Ser. No. 468,648 2 Claims.(Cl. 331-2) This invention relates to the automatic phase locking of aplurality of oscillators in an electromagnetic spectrum generator.

There exist radar applications which demand unique local oscillator andtransmitter signals. The usual transmitted signal assumes the form of apulsed, single frequency. The receiver local oscillator, in turn, is acontinuous, single frequency, offset in frequency from the transmittedenergy by a convenient intermediate frequency.

There are advantages, in some instances, in transmitting a spectrum ofphase locked frequencies and mixing the return with a local oscillatoroutput consisting of a spectrum of phase locked frequencies.

The present invention deals with the problem of generating thetransmitter and receiver spectrums.

An object of the invention is to provide a means for accuratelymaintaining phase lock between a plurality of frequencies in a spectrumgenerator.

Another object of the invention is to provide the above means withlittle changes in existing equipment and consequent small cost.

The objects of the invention are fulfilled by changing the responsecurve of an IF amplifier in the spectrum generator so that certainfrequencies, which might cause false phase locks, are amplified muchless than the desired intermediate frequency. Additionally, the mixer ofthe generator which feeds the IF amplifier has its inputs adjusted, inaccordance with the invention, to cause reduction of another cause offalse phase lock.

The invention may be best understood by reference to the drawings, inwhich:

FIGURE 1 shows an arrangement for generating a spectrum of frequenciesand wherein each frequency is separated from its neighbor by a discretefrequency and accurately phase locked to it,

FIGURE 2 shows the arrangement for a single output of the spectrumgenerator of FIGURE 1,

FIGURE 3 shows the response curve of the IF amplifier of FIGURE 2 asadjusted in accordance with the invention,

FIGURE 4 shows specific examples of inputs to, and output of, the mixerof FIGURE 2,

FIGURE 5 shows a general representation of the FIG- URE 2 mixer, and

FIGURE 6 shows another general representation of the FIGURE 2 mixer.

In FIGURE 1, there is shown a chain of three oscillators 10, 11 and12which it is desired to phase lock. This may be accomplished as by theinvention as described below. Each oscillator is frequency controlled bya respective D.C. amplifier 13, 14 or 15. The input to a D.C. amplifiersuch as 13 is the output of a phase detector 16, which detects the phasedifference between a low frequency reference 17 and the output of an IFamplifier 18. The IF of 18 is the difference between a high frequencyreference 19 and the output of the oscillator No. 1 (10), for thetopmost mixer 20, and difference between the frequency of the oscillatorNo. 1 (10) and oscillator No. 2 (11) is for the middle mixer 21 of FIG-URE 1, etc. A buffer amplifier such as 22 may be used between the lowfrequency reference 117 and each phase detector such as 16.

The chain of phase locked oscillators as shown in ited States PatentFIGURE 1 can be extended to include (within reason) an arbitrary numberof frequencies. There are definite problems lurking in the individualphase loops that can reduce the system to a tangle of related faultsunless carefully considered.

The arrangement shown in FIGURE 2 forces oscillator No. 1 to oscillateat a frequency (f f [(f +f is another possibility] and phase locked to fQualitatively, the operation becomes clear if numbers are assigned. Letf =5O0 mc., 71:10 mc. and when phase locked the frequency of oscillatorNo. 1 is 490 me. Let us assume the system is phase locked and start atoscillator No. 1 and trace around the loop. The 490 mc. is mixed inmixer 20 with 500 mc. yielding a 10 mc. difference frequency which inturn is amplified by the 10 mc. IF strip 18 and compared in phase to the10 me. reference (f in the phase detector 16. The resulting DC. isamplified by DC. amplifier 13 which in turn controls the oscillatorfrequency. Unfortunately, the system can also phase lock when oscillatorNo. 1 is separated by 5 mc. from the reference f Obviously, these modesof operation ruin the line spacing in the generated spectrum. The 5 and20 LIIIC. locks are called false lock. The obvious cure for the 20 mc.lock and one cause of the 5 me. lock is to simply shape the [F amplifierresponse such that the IF gain at 5 and 20 mc. (and resulting loop gain)is down about 40 db. It is not difficult to design the IF response asshown in FIGURE 3, and eliminate the 20 mc. lock and one cause of 5 mc.locks. However, there is a second source of 5 mc. locks that is moreillusive and will not yield to the measures described. This invention isconcerned with this fault.

Referring to FIGURE 4, assume that oscillator No. 1 is tuned to (orsweep thru) 495 me. and the mixer output is 5 me. (as describedearlier). If the second harmonic of 5 me. or 10 me. is of sufficientmagnitude, IF strip 1 8 most certainly will respond (IF strip 18 cannotdistinguish a true 10 mc. input from the second harmonic of 5 mc.) andphase locking will occur. The level of the second harmonic of 5 mc. isusually much less than the fundamental of the true 10 me. difference;however, the loop gain is sufficient to achieve phase lock at reduced 10me. levels. The standard procedure is to drive mixer 20 with a 10 to 1ratio of local oscillator power to signal power. Thus, the mixer outputretains the signal input characteristics diminished by the conversionefficiency. However, in this mode of operation, the second harmonic of 5me. still yield false locks. This mathematical description of FIGURE 5clearly exposes the problem and suggests the solution.

Referring to FIGURE 6, the characteristics of mixer 20, a non-lineardevice, may be represented by four terms of a power series as follows:

quency. When this input function is substituted into four terms of thepower series, the following results:

The terms of the power series have been expanded and the resultingfrequencies and coefiicients tabulated. For

example, the coefiicient and frequencies of the side band pair (c-s) and(c-i-s) is as follows:

coefficients 3a c s 3a.;c s

frequencies v= zco o+ 2 The the 10 me. output level is a function of 3as c 4. This latter term will yield a false lock if of sufliicentstrength. To eliminate the latter term, one solution is to operate themixer square law .retaining only the first two terms in the powerseries, i =a e +a e Clearly, as a approaches zero so does the term 3a sc 4, and the remaining source of false locks follows.

This involves biasing the crystals if crystal balanced strip line mixersare used. This scheme will work; however, the bias current levels arecritical (the current levels are different for each crystal although thecrystals are advertised as matched pairs) and the dynamic crystalimpedance is a function of bias current. Thus, the problem of retainingthe proper impedance match and balance in the strip line balanced mixerwhen operating square law is not any easy task.

If we reexamine the mixer output as a function of mixer inputs, a moreeffective solution is evident. The (cs) frequency output has thecoefficient while the 2(cs) frequency (the false lock frequency) retainsthe coefficient 3a s c /4. The power of the (c-s) frequently isapproximately proportional to C (the signal input to the balanced mixer)raised to the first power, while the power of the 2/(cs) frequency is afunction of C squared. Therefore, if we reduced the magnitude of C intothe mixer by a factor of 10, we reduce the power of the (cs) outputfrequency by but the 2(c-s) power is reduced by 100. Therefore, if weallow the ratio of mixer inputs s /c to assume a value of 100 instead of10, we diminish the power of the 2=(cs) term compared to the (c-s) termby a factor of 10.

The argument may be raised that the loop gain will be diminished since(c-s) is reduce-d by db. However, this gain is easily recovered in theIF strip where excess gain is available. In a transistor IF strip thenumber of stages is determined primarily by the skirt selectivity (afunction of the number of tuned circuits). If designed properly, theequivalent stable gain bandwidth available per stage is in excess ofthat needed.

Of course, the 2(cs) term will be amplified by the additional 20 db ofIF gain, but the generation of the 2(cs) frequency was diminished by 40db. This arrangement retains the original loop gain but attenuates thesource of false locks by 20 db which is sufficient to reliably preventfalse locks.

This arrangement requires no additional hardware to prevent false looksbut rather a more efficient use of existing circuitry.

While a specific embodiment of the invention has been disclosed,variations may be made in the frequencies employed or in the circuitelements used without departing from the invention. For example, adifferent IF would be used from that disclosed. Also, vacuum tube orother types of amplifiers could be used in place of the transistoramplifiers mentioned.

I claim:

1. A phase locked spectrum generator comprising a first oscillatorhaving a first frequency output, a second oscillator having a secondfrequency output, a third oscillator having a control input and a thirdfrequency output, wherein said third frequency is the difference betweensaid first and second frequencies, a mixer having plural inputs and anouput and with said second and third oscillators connected to inputs ofsaid mixer, the output of said mixer being the difference between saidsecond and third frequencies and equal to said first frequency, saidmixer output being connected to an intermediate amplifier having anoutput, said intermediate amplifier output and said first oscillatoroutput being connected to a phase detector having an output, connectingmeans between said phase detector output and the control input of saidthird oscillator, a fourth oscillator having a control input and afourth frequency output, an additional mixer means having plural inputsand an output, said third and fourth oscillators being connected toinputs of said additional mixer, the output of said mixer being thedifference between said third and fourth frequencies and equal to saidfirst frequency, said ad-- ditional mixer output being connected to anadditional intermediate amplifier having an output, said additionalintermediate frequency amplifier output and said first oscillator outputbeing connected to an additional phase detector having an outputconnected to an additional connecting means, said additional connectingmeans being also connected to the control input of said fourthoscillator.

2. The generator as defined in claim 1 wherein the ratio of amplitudesof the frequency outputs of said fourth and third oscillators connectedto the inputs of \aid additional mixer is to 1.

References Cited by the Examiner UNITED STATES PATENTS 2,786,140 3/1957Lewis 331 2 2,917,713 12/1959 Grauling 331 22 2,987,680 6/1961 Israel331 2 OTHER REFERENCES Terman Radio Engineers Handbook, 1943, McGraw-Hill Book Co., pp. 568, 569.

ROY LAKE, Primary Examiner.

JOHN KOMINSKI, Assistant Examiner,

1. A PHASE LOCKED SPECTRUM GENERATOR COMPRISING A FIRST OSCILLATORHAVING A FIRST FREQUENCY OUTPUT, A SECOND OSCILLATOR HAVING A SECONDFREQUENCY OUTPUT, A THIRD OSCILLATOR HAVING A CONTROL INPUT AND A THIRDFREQUENCY OUTPUT, WHEREIN SAID THIRD FREQUENCY IS THE DIFFERENCE BETWEENSAID FIRST AND SECOND FREQUENCIES, A MIXER HAVING PLURAL INPUTS AND ANOUTPUT AND WITH SAID SECOND AND THIRD OSCILLATORS CONNECTED TO INPUTS OFSAID MIXER, THE OUTPUT OF SAID MIXER BEING THE DIFFERENCE BETWEEN SAIDSECOND AND THIRD FREQUENCIES AND EQUAL TO SAID FIRST FREQUENCY, SAIDMIXER OUTPUT BEING CONNECTED TO AN INTERMEDIATE AMPLIFIER HAVING ANOUTPUT, SAID INTERMEDIATE AMPLIFIER OUTPUT AND SAID FIRST OSCILLATOROUTPUT BEING CONNECTED TO A PHASE DETECTOR HAVING AN OUTPUT, CONNECTINGMEANS BETWEEN SAID PHASE DETECTOR OUTPUT AND THE CONTROL INPUT OF SAIDTHIRD OSCILLATOR, AND FOURTH OSCILLATOR HAVING A CONTROL INPUT AND AFOURTH FREQUENCY OUTPUT, AN ADDITIONAL MIXER MEANS HAVING PLURAL INPUTSAND AS OUTPUT, SAID THIRD AND FOURTH OSCILLATORS BEING CONECTED TOINPUTS OF SAID ADDITIONAL MIXER, THE OUTPUT OF SAID MIXER GEING THEDIFFERENCE BETWEEN SAID THIRD AND FOURTH FRQUENCIES AND EQUAL TO SAIDFIRST FREQUENCY, SAID ADDITIONAL MIXER OUTPUT BEING CONNECTED TO ANADDITIONAL INTERMEDIATE AMPLIFIER HAVING AN OUTPUT, SAID ADDITIONALINTERMEDIATE FREQUENCY AMPLIFIER OUTPUT AND SAID FIRST OSCILLATOR OUTPUTBEING CONNECTED TO AN ADDITIONAL PHASE DETECTOR HAVING AN OUTPUTCONNECTED TO AN ADDITIONAL CONNECTING MEANS, SAID ADDITIONAL CONNECTINGMEANS BEING ALSO CONNECTED TO THE CONTROL INPUT OF SAID FOURTHOSCILLATOR.